Isengard consisted of a circular stone-wall surrounding a broad plain, in the center of which was the Tower of Orthanc. Isengard had only one gate, which faced south.

The river Angren (or Isen) began at Methedras behind Isengard, which also formed its northern wall. The other three sides were guarded by a large wall, known as the Ring of Isengard, which was only breached by the inflow of the river Angren at the northeast through a portcullis, and the gate of Isengard at the south, at both shores of the river.

Inside the tower of Orthanc

Isengard was, at first, a green and pleasant place, with many large trees and grass fields, fed by the river until it was dammed. After Saruman's defeat, however, Treebeard and the rest of the Ents made it a vivid green area again, after destroying the wall, naming it the "Treegarth of Orthanc".

and the last warden of Orthanc was recalled to Minas Tirith. Isengard remained guarded by a small company, led by a hereditary captain, much as happened at Aglarond at the time.

After Calenardhon was given to the Éothéod by Cirion, Steward of Gondor, and became Rohan, Isengard remained part of Gondor, and the company of Aglarond removed to the northern fortress, although the rest of Gondor almost forgot about it. The small guard intermarried much with the Dunlendings, until the place became a Dunlending fortress in all but name. Orthanc, however, remained closed, as the Steward of Gondor still held the keys. Isengard soon came all but deserted.

During the rule of Rohan's KingDéor it turned out that Isengard had become openly hostile to the Rohirrim. Using Isengard as their base, the Dunlendings continued to raid Rohan during Déor's son Gram's rule, until during the rule of Gram's son Helm Hammerhand a Dunlending lord, Freca and his son Wulf nearly managed to destroy the Rohirrim. The Rohirrim eventually won, taking Isengard, and guarding it for Gondor.

During the time of the Steward of Gondor Beren, Saruman the White suddenly reappeared from the East, and he offered to guard Isengard. Beren gladly gave him the keys to Orthanc, and Saruman settled there. After him the valley became known as Nan Curunír, or "Wizard's Vale".

During the War of the Ring, Isengard was Saruman's base of operations against the Rohirrim, and he defiled the valley, cutting down its trees and grass and replacing them with stone and machinery. He dammed and diverted the Angren, possibly for use as hydropower. Isengard's valley was honeycombed with deep pits, used for breeding Uruk-hai and housing orcs, men and wolves and smithying weapons. Isengard became home to countless Orcs and Dunlendings -- a sizable private army. Eventually, an army of Ents and Huorns led by Treebeard of Fangorn attacked Isengard, wrecking all of Saruman's industries, although they could not force entry to the Tower of Orthanc.

Isengard at full military power

The HobbitsMeriadoc Brandybuck and Peregrin Took as the new "doorwardens" received ThéodenKing of Rohan, Aragorn, and Gandalf at the gates. Saruman was confronted but refused to beg for pardon, and he was released into Treebeard's guard together with his servant Gríma Wormtongue. Saruman abandoned Isengard after Sauron's defeat, playing off Treebeard's hatred of caging any living thing in order to manipulate him into letting him leave. It was planned by Sauron that Isengard would be given to his loyal ambassador, the Mouth of Sauron, after Saruman's demise.

Under the Ent dominion, the ringwall was torn down and the Ents planted the Watchwood there and renamed the new forest the Treegarth of Orthanc. Saruman's pits and caverns were filled, and the land was healed. The Valley was now filled with trees and a small lake was constructed near Orthanc. The valley was given to the ents by Aragorn, to do with it as they wished, granted that they keep watch upon Orthanc and preventing anyone without permission to enter it.